Cyclic sewing machines for industrial use, such as those used to make bar tacks, buttonholes, and other stitch formations that are followed by cutting the thread by means of an underbed thread trimmer must be arranged so that the end of the bobbin thread is long enough to be engaged by the needle thread in the formation of the next set of stitches. In addition, the end of the bobbin thread must be in the right position for this to happen.
In some bobbin cases, the thread emerges from the cylindrical side of the case underneath the leaf spring that closely follows the curvature of the cylindrical side. At the end of the leaf spring, the thread is free of further constraints. When a bobbin case of that type is used in a cyclic sewing machine, or at least a sewing machine that has an underbed thread trimmer, the cutting mechanism that severs the thread does so at a location approximately an inch or so away from the point at which the thread emerges from under the leaf spring. The severed end of the bobbin thread at the location of the cutting knife or knives is both axially and angularly displaced from the point of emergence of the thread from under the leaf spring, and it is possible for the short length of thread between the leaf spring and the severed end to droop between the time the thread is severed and the time the material to be sewen is repositioned to make the next set of stitches. If the end of the thread is long enough it occasionally droops far enough to extend into the path of rotation of the rotary hook in which the bobbin case is located, and when that happens, the end of the thread may be pulled part of the way around the circumference of the bobbin case between the cylindrical portion of the case and the inner wall of the hook. In that event, the bobbin thread will not engage the needle thread, and no stitches can be formd until the operator opens up the mechanism to retrieve the end of the bobbin thread and place it in a proper position to be caught by the needle thread.
A number of forms of bobbin cases have been made heretofore with integral or attached means to hold the thread at a point beyond the end of the leaf spring so that the free end of the bobbin thread will be closer to the proper position to be engaged by the needle thread in forming the next set of stitches. In some instances, such as in U.S. Pat. No. 2,694,373, an additional bracket has been attached by means of a small screw to provide a channel for the bobbin thread. In other types of bobbin cases, a curled pigtail of wire has been soldered to the bobbin case to form an eye through which the bobbin thread is led. In further types of bobbin cases, such as in U.S. Pat. No. 3,486,473, a machined element has been added to the wall of the case to form a tunnel through which the thread can be inserted. As an alternative to providing guide means on the bobbin cases, it has been proposed previously to form grooves along the surface of bobbin case baskets as in U.S. Pat. No. 3,074,367 or to form tunnels through bobbin case baskets as in U.S. Pat. No. 944,740 or to provide a slot closed by spring thread retaining means as in U.S. Pat. No. 1,197,611.
All of the foregoing bobbin cases and baskets are complex to construct and do not place the channel through which the thread must extend directly in line between the point of emergence of the thread from under the leaf spring and the point of engagement between the bobbin thread and the needle thread.